Color display devices such as color TVs, color monitors and the like usually represent colors by additive color mixture of RGB colors (i.e., red, green and blue). In general, pixels of a color display panel each include red, green and blue sub-pixels in correspondence with the ROB colors. Such a display is referred to also as a “three primary color display device”. To a display panel of the three primary color display device, YCrCb (YCC) signals which can be converted into RGB signals are input, and based on the YCrCb signals, the luminance values of the red, green and blue sub-pixels are changed. Thus, various colors are represented. In the following description, the luminance value (luminance level) of a sub-pixel corresponding to the minimum gray scale level (for example, gray scale level 0) is represented as “0”, and the luminance value of a sub-pixel corresponding to the maximum gray scale level (for example, gray scale level 255) is represented as “1”. The luminance values of the red, blue and green sub-pixels are each controlled in the range of “0” to “1”.
When the luminance values of all the sub-pixels, i.e., the red, green and blue sub-pixels are “0”, the color displayed by the pixel is black. By contrast, when the luminance values of all the sub-pixels are “1”, the color displayed by the pixel is white. Many of recent TVs allow even a user to adjust the color temperature. In such a TV, the color temperature is adjusted by fine-tuning the luminance value of each sub-pixel. Here, the luminance value of a sub-pixel after the color temperature is adjusted to a desired level is represented as “1”.
Hereinafter, with reference to FIG. 65, changes of the luminance values of the sub-pixels in the three primary color display device will be described. Here, the color displayed by the pixel changes from black to white via green. The luminance value of each sub-pixel is changed so as to maximize the chroma at each lightness of the color displayed by the pixel. In the following description, the hue of the color displayed by the green sub-pixel is represented as the “hue (G)” or simply “(G)”. The hue (G) is defined by the green sub-pixel.
FIG. 65 shows the relationship between the change of the luminance value of each sub-pixel and the change of the color displayed by the pixel in the three primary color display device. FIG. 65(a) shows the change of the color displayed by the pixel, and FIG. 65(b) shows the changes of the luminance values of the red, green and blue sub-pixels.
In an initial state, the color displayed by the pixel is black, and the luminance values of the red, green and blue sub-pixels are “0”. First, the luminance value of the green sub-pixel starts to be increased. As the luminance value of the green sub-pixel increases, the chroma and the lightness of the color displayed by the pixel increase. When the luminance value of the green sub-pixel reaches “1”, the chroma of the color displayed by the pixel is maximized at the hue (G). In the following description of this specification, among the colors displayed by the display device, a color having a highest chroma at each hue angle on an a*b* plane of an L*a*b* colorimetric system will be referred to as the “optimal color”.
When reaching “1”, the luminance value of the green sub-pixel is kept “1”. Then, in order to further increase the lightness of the pixel, the luminance values of the other sub-pixels (i.e., the red and blue sub-pixels) start to be increased. In this step, the luminance values of the red and blue sub-pixels increase at an equal rate. By increasing the luminance values of the red and blue sub-pixels at an equal rate, the lightness of the pixel can be increased without changing the hue (G) almost at all. When the luminance values of all the sub-pixels become “1”, the color displayed by the pixel is white. In this manner, in the three primary color display device, the color displayed by the pixel is changed from black to white via the optimal color of the hue (G) by first lighting up the green sub-pixel and then lighting up the red and blue sub-pixels.
Now, with reference to FIG. 66, changes of the luminance values of the sub-pixels in the three primary color display device in the case where the color displayed by the pixel changes from black to white via cyan will be described. Cyan is represented by the green and blue sub-pixels. Here, the hue of the color represented by making the luminance values of the green and blue sub-pixels approximately equal to each other is represented as the “hue (C)” or simply “(C)”.
FIG. 66 shows the relationship between the change of the luminance value of each sub-pixel and the change of the color displayed by the pixel in the three primary color display device. FIG. 66(a) shows the change of the color displayed by the pixel, and FIG. 66(b) shows the changes of the luminance values of the red, green and blue sub-pixels.
In an initial state, the color displayed by the pixel is black, and the luminance values of all the sub-pixels are “0”. First, the luminance values of the green and blue sub-pixels start to be increased to “1” at an equal rate. When the luminance values of the green and blue sub-pixels reach “1”, the color displayed by the pixel is the optimal color of the hue (C).
When reaching “1”, the luminance values of the green and blue sub-pixels are kept “1”. Then, in order to further increase the lightness of the pixel, the luminance value of the red sub-pixel starts to be increased. By increasing the luminance value of the red sub-pixel while keeping the luminance values of the green and blue sub-pixels “1”, the lightness of the pixel can be increased without changing the hue (C). When the luminance values of all the sub-pixels become “1”, the color displayed by the pixel is white. In this manner, in the three primary color display device, the color displayed by the pixel is changed from black to white via the optimal color of the hue (C) by first lighting up the green and blue pixels and then lighting up the red sub-pixel.
As can be understood from the above, the three primary color display device can represent various colors by changing the luminance value of each sub-pixel. However, the range of colors reproduced by a general three primary color display device is narrower than the range of colors perceivable by the human. Therefore, in order to enlarge the range of colors reproducible by the display device, it is conceived to increase the thickness of the color filter to raise the color purity or to use a high color purity LED. However, such techniques decrease the brightness and thus reduces the light source efficiency.
Recently, as opposed to the three primary color display device as described above, display devices using additive color mixture of four or more primary colors have been proposed (see, for example, Patent Documents 1 through 3). Such a display device which performs display using four or more primary colors is referred to also as the “multi-primary color display device”. Patent Documents 1 and 2 disclose a multi-primary color display device including pixels having red, green, blue, yellow, cyan and magenta sub-pixels. Patent Document 3 discloses a multi-primary color display device including another red sub-pixel instead of the magenta sub-pixel.